1Heat gas or charcoal grill. In small bowl, stir orange juice, shallots, honey, oil, mustard, 1/8 teaspoon salt and 1/8 teaspoon pepper with whisk until well blended; set aside.
2Sprinkle salmon with 1/2 teaspoon salt and 1/4 teaspoon pepper. Carefully brush oil on grill rack. Spray salmon and onion slices with cooking spray. Place salmon and onion on grill over medium heat. Cover grill; cook 10 minutes, turning once, until fish flakes easily with fork and onion is tender. Remove skin from fish.
3Cut onion into bite-size chunks; break fish into chunks. Divide salad greens among serving plates; top with grapefruit sections, onion and fish. Drizzle with vinaigrette.